Precast 3 Floors

Precast & Prestressed Floors

and Composite Slabs

The Design and Construction of Precast Concrete Structures

Precast & Prestressed Floorsand Composite Slabs

Hollow core floor units & slab fields

Double tee units

Half-slab (precast + insitu topping)

Composite floors

Load v span data

Spreadsheets for design of units and slab fields

Dutch trade association claim fixing rates of 2000 sq.m per week

40 x 11 feet wide hollow core onto precast walls at MGM hotel, Las Vegas, 1992

European production = 25 million sq. m per year

75% is 150 – 250 mm deep

Double tee units – twice the price but up to 4 x capacity than hollow core

Prestressed half-slab popular for housing and awkward shapes

Propping required over 5 m

PRESTRESSED HOLLOW CORE FLOOR UNITS

400 – 3600 mm wide; typically 1200 mm

90 – 730 mm deep; typically 150, 200, 250, 300 mmself weight 1.5 to 5 kN/sq.mvoid ratio 40 – 60 % of solid sectionspans 6 – 20 m (economical range)

Longitudinal pretensioning strand or wire

1195 mm

No shear or torsion links

30 mm flanges and webs

Shear key profile

Are the deeper units ‘beams’ or ‘slabs’?

Should they abide by normal RC rules?

Extrusion or slipformed

100-150 m long bed; no-slump mix grade C50-60

Extrusion - rotating screws turn opposite hands

Extrusion – circular mandrels make the holes

Slipforming – shear compactor (hammers down flanges and webs)

Sliding motion to and fro

Curing temperature contours (c Branco, Lisbon)

Sliding motion to and fro

Splitting cracks due to sawing restraints as prestress in transferred

At 16-18 hours, circular saw cuts to length +10 to -15 mm

The new carousel system for continuous production of 2 x 1.2 m wide units

e.g. Spancrete, USA

Bison, UK (2006)

Italian machinery

Factory 500 m long, including labs and prep

The new carousel system for continuous production of 2 x 1.2 m wide units

Cement and admixtures

10-14 mm coarse and fine agg

Mixer & re-cycled slurry

Tensioned wire

Steam curing under coversID chip marker

Sawing room

Moving steel beds collect the tensioning wires

1.8 m per

minuteSteel plates move across onto roller beds

5 mm indented wire tensioned over 120 m

Continuous concrete delivery for 2 x 120 m long extrusions

Production at the rate of 4.8 sq. m per minute, yielding 1200 sq. m per day

Steam curing under cover for 16 hours. Transfer strength = 40 N/mm2

Identification chip is automatically glued onto top of unit

Units cut to length + 10 mm accuracy-

Final lifting into stockyard

400 – 450 deep units have a new market for 16 m long clear span car parks

Too much plasticiser !! in 450 deep units

Actually air-entrainment agent is used by several producers as a plasticiser

Single storey supermarket podium and car park

Span/depth ratio = 40

Italian variation (c. ASSAP)

600 mm wide prestressed units in Budapest

7-wire helical strand (1750 MPa) gives good bond in the important transmission zone

Locks in

Strand pull-in:

An important indicator of success

Should be about 1 mm, irrespective of length

Theoretical pull-in limit for zero prestress

PL

Use a linear scale between the extremes

AE

700 mm deep units in Italy, often used at 25 m span for tunnel cut-and-cover

For units > 500 mm deep, mesh is rolled out to reinforce the outer webs

Bearing onto neoprene or mortar for spans more than about 15 m onto insitu or masonry

20 m long ASSAP unit

M = (f + f ) Zsr bc ct b

Section Analysis for prestressed

- Pretension and losses (about 18-25%)

- Service moment (bottom tension critical)

- Ultimate moment (usually > service x 1.5)

- Ultimate shear uncracked & flexurally cracked

- End bearing and transmission length

- Deflection and camber (long-term, creep)

- Live load deflection after installation

M = 0.95 f A (d-d )ur pu ps n

V = 0.67 b h f + 0.8 f fco v t cpx t 2

Allowable span

Impo

sed

load

Span

Impo

sed

load

Bearing limit

Handling limit span/depth = 50

Span

Impo

sed

load

Bearing

Shear

Service moment

DeflectionHandling

Service moment control

Deflection control

Possibly shear ?

Routine bending tests to 25% overload with 95% recovery

Flexural cracks extend rapidly through the section due to narrow webs

Approx

200 mm spacing

Effective stiffness changes with increasing load

Load

Deflection

EI = EI + (EI - EI ) (M/ M )eff c u c crack2 to 6

Watch out for flexural-shear (V ) failure !cr

Vco

V = 0.55(1-loss) v b d + M Vcr c v o uMu

Vcr

Decompression point

UDL

SF diagram

Vcr failure here

P

Failure of solid prestressed unit during pouring of insitu topping

Cover to the tendons

100 mm50-55 mm

Shear failure in 400 deep units (c. Engstrom, Sweden)

Shear searches out the weakest web

Shear capacity of single webs more reliable

Shear bond crack stops

Aswad tests on edge loads (PCI JOURNAL)

Very brittle in this mode !

Lateral load distribution

LINE LOADS PAR’L TO SPAN

LATERAL DISTRIBUTION

POINT LOAD PER UNITS

SIMPLY SUPPORTED

TIE STEEL

Shear keys in longitudinal joints

Edge loads

distribution

FIP

data

(Van

Ack

er 1

984)

Edge loads

distribution

FIP

data

(Van

Ack

er 1

984)

30%

2Reinforced Hollow Core: 600 mm wide

Housing and office spans up to about 5.5 m

Self weight approx 270 kg/m

Must use partial cracked stiffness and quasi-permanent live load (φ = 0.3) for deflections

Made upside down. 6 no. T8-T20 bars

Two pass of concrete, grade C40

Cores withdrawn immediately. Cycle takes less than 3 minutes. Equipment maintenance approx. 50% of extrusion m/c costs

24 hour drying and humid curing

2400 – 3000 mm wide300 – 2000 mm deep; typically 400 - 800 mmself weight 2.6 to 10 kN/sq.mvoid ratio 70 – 80 % of solid sectionspans 8 – 30 m (economical range)

DOUBLE TEE FLOOR UNITS

Double tee units – mostly prestressed, but RC if manufacturer prefers

Bearing pads required 150 x 150 x 10

Double tee long-line casting

Prevent settlement cracks at the top of the web

Double tee - ‘concrete train’ in Germany

Self topped units with 120 mm thick flanges

Double tee – self compacting concrete pour.

1 batching + 4 workers = 240 sq.m per day equates to product cost / salary ratio = 20

Half joint box and confinement U bars

Half joint lowers the centroid of the floor plate

Site weld to adjacent flange

Make a small saw cut to prevent spalling

Composite floors required for double tee, but optional for hollow core

Surface laitence due to cutting slurry

More than 0.5 mm thick

50 mm minimum at the highest point, increasing (with slab and beam cambers) to about 80 mm

Composite design – 2 stage approach

MINIMUM STRUCTURAL DEPTH FOR

SAME LOAD AND SPAN CONDITIONS

FLOOR SPAN

BEAM SPAN= 1.0 TO 1.5

6 – 10 m

6 - 1

5 m

Vibrations and Natural Frequency ofPrecast Concrete Floor Elements

Offices and industrial buildings -machines, vibrations through the ground, footfall

Sports halls -human activities (aerobic, jumping, dancing)

Concert halls

Grandstands -human activities (jumping, stamping)

Vibration & natural frequency The most important vibration property of a floor is its natural frequency fn, or frequency of its first mode.

Vibration & natural frequency

Vibration & natural frequency

Egenfrekvenser DT-elementer

02

468

1012

1416

2 4 6 8 10 12 14 16 18 20Spennvidde [L [m]

Frek

vens

f [H

z]

DT-200/50DT-300/50DT-400/50DT-500/50DT-600/50DT-700/500DT-800/500Grense

Vibration & natural frequency spreadsheet

Summary

Strength v economic cost indicator

Depth Cost index Strength index* Popularity

Hollow 150

Core 200 1 1 1

300

400

Double 500

Tee 800

* Based on service moment of resistance

Summary


Depth Cost index Strength index* Popularity

Hollow 150

Core 200 1 1 1

300

400

Double 500

Tee 800

* Based on service moment of resistance

Approx 120 kNm per unit

Approx £25 supply, £30 fixed per sq.m

Summary


Depth Cost index Strength index Popularity

Hollow 150 0.9 0.65 0.5

Core 200 1 1 1

300

400

Double 500

Tee* 800

* + 75 mm structural topping

Summary


Depth Cost index Strength index Popularity

Hollow 150 0.9 0.65 0.5

Core 200 1 1 1

300 1.3 1.9 0.1

400 1.6 3.0 0.3

Double 500 2.0 3.0 0.2

Tee 800 3.5 5.8 0.1

Summary


Depth Cost/Strength ratio

Hollow 150 1.40

Core 200 1

300 0.68

400 0.53

Double 500 0.67

Tee 800 0.60

Precast 3 Floors

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